linux-kernel - Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

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Date:   Wed, 27 May 2020 13:14:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Naohiro Aota <naohiro.aota@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Kristof Beyls <Kristof.Beyls@....com>, victor.campos@....com
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

On Wed, May 27, 2020 at 12:28 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2020-05-27 18:55, Nick Desaulniers wrote:
> > On Wed, May 27, 2020 at 6:45 AM Robin Murphy <robin.murphy@....com> wrote:
> >>
> >> On 2020-05-26 18:31, Nick Desaulniers wrote:
> >>> Custom toolchains that modify the default target to -mthumb cannot
> >>> compile the arm64 compat vdso32, as
> >>> arch/arm64/include/asm/vdso/compat_gettimeofday.h
> >>> contains assembly that's invalid in -mthumb.  Force the use of -marm,
> >>> always.
> >>
> >> FWIW, this seems suspicious - the only assembly instructions I see there
> >> are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the
> >> -march=armv7a baseline that we set.
> >>
> >> On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and
> >> built a Thumb VDSO quite happily with Ubuntu 19.04's
> >> gcc-arm-linux-gnueabihf. What was the actual failure you saw?
> >
> >  From the link in the commit message: `write to reserved register 'R7'`
> > https://godbolt.org/z/zwr7iZ
> > IIUC r7 is reserved for the frame pointer in THUMB?
>
> It can be, if you choose to build with frame pointers and the common
> frame pointer ABI for Thumb code that uses r7. However it can also be
> for other things like the syscall number in the Arm syscall ABI too.

Ah, right, with -fomit-frame-pointer, this error also goes away.  Not
sure if we prefer either:
- build the compat vdso as -marm always or
- disable frame pointers for the vdso (does this have unwinding implications?)
- other?

> I
> take it Clang has decided that writing syscall wrappers with minimal
> inline asm is not a thing people deserve to do without arbitrary other
> restrictions?

Was the intent not obvious? We would have gotten away with it, too, if
wasn't for you meddling kids and your stupid dog! /s
https://www.youtube.com/watch?v=hXUqwuzcGeU
Anyways, this seems to explain more the intentions:
https://reviews.llvm.org/D76848#1945810
+ Victor, Kristof (ARM)
-- 
Thanks,
~Nick Desaulniers